I have one location using at&t dsl that connects to the office vpn correctly but the speed is unacceptable. The internet speed is 2.5 mbps & when connected to the vpn the internet speed is unaffected. When I move the laptop to other wifi points - house, hotel cigar shop - everything works fine.

3 Answers
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Usually DSL modems are configured by the ISPs to have an unbalanced ratio so that downloads will be much faster than uploads. The reason for this is that people doing things like web browsing (which is what most users appear to do most of the time with their internet connections), and sending a request to a web server requires very little data compared to what often gets downloaded (when viewing videos, and listening to music or radio stations, this becomes quite pronounced because one simple request to a web server starts a steady and prolonged stream of data downloading).

So, where a ratio might be 10:1, a VPN would likely be effected because data transfer needs may be bi-directional (e.g., load a document, edit it, then save it) in nature unlike most web applications [that don't involve uploading].

You'll need to contact your ISP to find out what your specific ratio is because it may vary between ISPs. Chances are that the first-level support probably has no idea how to answer this question either, so you will probably have to get your question escalated to a higher level of technical support. (The "Network Operations Centre" might be who ends up answering your question.)

Some ISPs may provide an upgrade option (usually with a higher cost) to provide higher speed access. The cable providers usually do, but for DSL it's difficult to say because there are some known hard limits, and so you might have to look at other options that are not DSL based if you're already maxing out the technology.

The modems have nothing to do with whether a circuit is synchronous or asynchronous, its the technology type. ADSL is asynchronous so the upload will always be much less, SDSL and IDSL are synchronous so both will be equal. The modem has nothing to do with it. ADSL2 provides the fastest possible upload speed on an asynchronous DSL connection, and that is 1.4 mbps. Your ISP cannot change this. If you need faster upload speed (and with a VPN, you probably do), you should go with cable, or get a good solid low latency connection like a T1 (latency will kill a VPN).
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MaQleodMar 22 '11 at 15:28

@andrejako: lack of sleep made me use ATM terminology instead... Asymmetrical is what I meant, thanks for the correction there. As for the 1.4, that is an older standard (ITU G.992.5) but its also the most common standard that most ISPs will offer. You are right that if the ISP as upgraded to ITU G.992.5 Annex M, then they might offer higher uploads, but anything over 1.4 will be very dependent on distance of the copper run (you'll need to have about 8000 or less feet between you and the CO)
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MaQleodMar 22 '11 at 22:35

@MaQleod Indeed. Another problem I've seen with ISPs is uneven distribution of equipment, so some ISPs do not offer higher upload speeds to customers who do have technical capabilities because most of the other costumers won't be able to achieve them.
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AndrejaKoMar 22 '11 at 22:39

The VPN Connection and the DSL speed can also effect by the distance, and OS you have. Sometimes, your OS Platform can slow your internet speed or increase the internet speed. Also, the vpn connection from your home to your office can cause lag due to the following things:

VPN location first set

Web Traffic

Users on the VPN Connection
Now, what does the VPN location first set have to do with anything?
Well as for routers, you have to be in a certain area to get the Wi-Fi Connection.
Anyone that is using the VPN Connection is decreasing the speed on the VPN Connection. For example, if your office had up to 105 Mbps that would be pretty fast. But for 1 user, they have access to have 105 Mbps Via Ethernet Connection. Now, if 2 more users connected to that VPN Connection. There is a possible chance you lose 9.4% of your internet speed.

So really,
Yes. Connecting to the VPN connection can increase internet speed.
But uses more web traffic as well. And yes, Randolf is also correct as well with the ratio.